Diagnosing a Bad Coolant Temperature Sensor

[79er]

I've reviewed a bunch of threads on idle and starting issues and found that for some folks replacing the CTS is the answer, for others, no. But I haven't found any info on actually diagnosing a bad CTS as opposed to shelling out the bucks to replace and hope it does the trick. Is an ohmmeter check a reliable indicator, or are there other issues that a simple resistance check won't reveal? If an ohmmeter check is reliable, where can I find a temperature/resistance chart so as to make an informed decision if the CTS is good or going out of range? I'm looking for values appropriate to the CTS in 90-94 models; and if anyone has knowledge of how similar (or different) the resistance values for temp sensors over the years are, I'm sure others would find that useful as well.

 

Woody

[Manarius]

A bad coolant temperature sensor is something that is really easy to diagnose without using a multimeter. If a warm car continues to turn over and not start (assuming everything else is working correctly), the coolant temperature sensor is the culprit.

[keltik]

Also you should be able to unplug it and the car should start as normal. If removing the sensor from the equation fixes your problem - then thats a solid diagnoses. The ECU will just guess the temp and should still start....well mine did. This method works for MAF sensors too.

[Cougar]

Using an ohmmeter to check the resistance is a very good method to see if the sensor is working as it should be. You could also watch the voltage change as the engine warmed up. If the sensor isn't working as it should be then there will be little or no change in the values.

 

I don't have the specs for your model year but the sensor resistance may be around 300 ohms when it as normal operating temperature. There may also be about a 8 to 1 factor in the resistance when it is cold and then at operating temperature. The sensor may be about 3,500 ohms when cold.

[keltik]

My 1995 FSM states the coolant temp sensor should be diagnosed in the following manner:

 

1) Make sure the fault is with the sensor not the harness connecting to the sensor

 

2) Replace sensor with Subaru Genuine part

 

Im paraphrasing here of course but basically thats all ya get. No resistance values are given for this particular sensor and i checked a fair whack of the manual.

[frag]

Haynes:

 

Temp in °F Resistance (ohms)

212 ----------176

194 ----------240

176 ----------332

158 ----------458

140 ----------668

112 ----------972

122 ----------1182

104 ----------1458

95 -----------1800

86 -----------2238

76 -----------2795

68 -----------3520

58 -----------4450

50 -----------5670

40 -----------7280

32 -----------9420

[2X2KOB]

For future reference, here's the Coolant Temp Sensor data for a 2000 OBW with an EJ252 2.5, from the FSM:

[79er]

....I've reviewed a bunch of threads on idle and starting issues...

 

Thanks to all who replied. Guess I was too vague on the decription--I've been experiencing a high (2200-2500 rpm) idle on cold start on these mild summer days and infrequently a high idle that will only sloowly drop to under 1000 after running at sustained highway speeds (65-75). Most forum members commenting on this problem finger either the CTS or (for my older 92 Legacy) the air bypass valve (I'm assuming the term IAC refers to the same metered air bleed on later engines). In any event, some have found replacing the CTS the solution, others have not. I was looking for a way to narrow the problem without putting money into something not needed. My CTS is functional (engine starts without effort, idles a little high at 900 after warmup), I was just wanting to see if the resistance values were tracking with what the ECU expects to see for a given temperature, or skewed from what they should be. This morning I checked cold (62 degrees) and operating temps and found values of 1183 ohms and 62 ohms, respectively. That kinda ruled out the values in the Haynes guide quoted above, so I figured I'd bite the bullet and visit the local pull & save and see if I could find a couple of sensors to measure as a comparison. Found two, one from an '89 and another from a '93 (note: the 89-94 engines have a temp sensor that takes a 12 mm socket or wrench, not a 19mm, so probably a different supplier and different characteristics from the 19mm version. Anyway, here are the results for anyone in future that might need a rough reference for this style of coolant temp sensor--average of two sensors (with help from my freezer and microwave):

 

Temp Degrees F. Ave resistance (ohms)

10--------- 7370

40--------- 2275

62--------- 1205

71---------- 908

103--------- 365

118--------- 252

147--------- 102

165---------- 66

 

So I guess I'll be fussing with the air bypass control, though I gave it the "easy" treatment of a few shots of carb cleaner--it only intensified the problem--even higher startup idle than before the treatment. Anyone have new ideas to contribute? I figure there's gotta be an answer, though I recall reading at least one thread from a forum member with the same problem who replaced the CTS without any effect on the high idle problem; the thread kinda petered out after that with no resolution. Thoughts?

 

Woody

[keltik]

the air bypass valve (I'm assuming the term IAC refers to the same metered air bleed on later engines).

 

Technically the air bypass valve is found on turbo cars only and vents boost pressure back to the low pressure side of the intake (pre-turbo) when the throttle plate is closed.

 

The Idle air control valve or just idle control valve is the one were talking about here.

 

My new theory: (may only apply to JDM cars as ive never seen any USDM stuff)

The IACV has a small bi-metallic spring in the bottom of it, this is fed coolant from the engine top crossover pipe and helps the valve set its workable range from when the engine is cold to when its hot. If this spring isnt getting a coolant feed - the engine will idle high when its hot. The rubber hoses that feed this part have a very small internal diameter and can gunk up easily, so it could be worth whipping the valve off and checking your getting a coolant flow to it.

 

If you run a D-check on the car do you get any codes?

[Olnick]

So I guess I'll be fussing with the air bypass control, though I gave it the "easy" treatment of a few shots of carb cleaner--it only intensified the problem--even higher startup idle than before the treatment. Anyone have new ideas to contribute? Woody

 

I think at this point I'd pull the IACV and give it a thorough cleaning, including the hoses. Doesn't cost anything other than time and cleaner. Did mine a couple of years ago and have had a very smooth and predictable idle since then. ('Course I replaced the coolant temperature sensor too sometime back and that solved a lot of problems, so who knows for sure?!!)

 

Anyway, we had some threads talking about the IACV so it might be worth a search.

 

Good luck!